Facile synthesis of 5,10,15-hexaaryl truxenes: Structure and properties

Article abstract of DOI:10.1021/ol200227w

A new synthesis of 5,10,15-hexaaryltruxene derivatives has been developed. Experimental observations and theoretical calculations confirmed that the presence of aryl substituents on the sp³-hybridized bridge carbon atoms had an effect on the ph

Full text of DOI:10.1021/ol200227w

ORGANIC
LETTERS
2011
Vol. 13, No. 7
1714–1717
Facile Synthesis of 5,10,15-Hexaaryl
Truxenes: Structure and Properties
Min-Tzu Kao,† Jia-Hong Chen,† Ying-Ying Chu,† Kuo-Pi Tseng,† Chia-Huei Hsu,† Ken-
Tsung Wong,*,† Che-Wei Chang,‡ Chao-Ping Hsu,*,‡ and Yi-Hung Liu†
Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan, and
Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan
kenwong@ntu.edu.tw; cherri@sinica.edu.tw
Received January 25, 2011
ABSTRACT
A new synthesis of 5,10,15-hexaaryltruxene derivatives has been developed. Experimental observations and theoretical calculations confirmed
that the presence of aryl substituents on the sp3-hybridized bridge carbon atoms had an effect on the photophysical properties of the truxene core.
Interestingly, the cage compound 11 possessing a distorted truxene core was verified by X-ray diffraction analysis; the influence of the peripheral
aryl substituents on the photophysical properties of the truxene core was diminished because of its molecular rigidity.
Truxene, an interesting C3-symmetric coplanar struc-
ture, has emerged as an important core block for the
preparation of novel starburst oligomers1 and dendrimers2
that have potential applications in various systems,
including liquid crystals,3 two-photon absorption
systems,4organic light-emitting diodes,5 photovoltaics,6
light-harvesting systems,7 and field effect transistors.8
The physical properties of truxene-based materials can
be manipulated by extending the π-conjugation emanating
from the truxene core and also by introducing rigid
peripheral units on the sp3-hybridized carbon bridges.9
Truxene-centered materials are generally derived through
modification of the truxene core, which is typically synthe-
sized through the condensation of indanones.10 The per-
ipheral substituents attached to the saturated C5, C10, and
C15 carbon bridges are typically introduced by the alkyal-
tion of truxene or nucleophilic addition of truxene-5,10,15-
trione.11 To the best of our knowledge, no facile syntheses
of 5,10,15-hexaaryl-substituted truxene derivatives have
been reported previously. Herein, we describe an efficient
synthetic pathway toward such compounds and report
† National Taiwan University.
‡ Academia Sinica.
(1) (a) Pei, J.; Wang, J.-L.; Cao, X.-Y.; Zhou, X.-H.; Zhang, W.-B. J.
Am. Chem. Soc. 2003, 125, 9944. (b) Zhang, W.-B.; Jin, W.-H.; Zhou, X.-
H.; Pei, J. Tetrahedron 2007, 63, 2907. (c) Cao, X.-Y.; Hong, Z.; Zhang,
W.; Lu, H.; Pei, J. J. Org. Chem. 2005, 70, 3645. (d) Chen, Q.-Q.; Liu, F.;
Ma, Z.; Peng, B.; Wei, W.; Huang, W. Synlett 2007, 20, 3145.
(2) (a) Cao, X.-Y.; Zhang, W.-B.; Wang, J.-L.; Zhou, X.-H.; Lu, H.;
Pei, J. J. Am. Chem. Soc. 2003, 125, 12430. (b) Jiang, Y.; Lu, Y.-X.; Cui,
Y.-X.; Zhou, Q.-F.; Ma, Y.; Pei, J. Org. Lett. 2007, 9, 4539. (c) Jiang, Y.;
Wang, J.-Y.; Ma, Y.; Cui, Y.-X.; Zhou, Q.-F.; Pei, J. Org. Lett. 2006, 8,
4287.
(3) (a) Chandrasekhar, S.; Ranganath, G. S. Rep. Prog. Phys. 1990,
53, 57. (b) Sandstrom, D.; Nygren, M.; Zimmermann, H.; Maliniakt, A.
J. Phys. Chem. 1995, 99, 6661.
(4) Zheng, Q.; He, G. S.; Prasad, P. N. Chem. Mater. 2005, 17, 6004.
(5) (a) Cao, X.-Y.; Liu, X.-H.; Zhou, X.-H.; Zhang, Y.; Jiang, Y.;
Cao, Y.; Cui, Y.-X.; Pei, J. J. Org. Chem. 2004, 69, 6050. (b) Xia, H.; He,
J.; Xu, B.; Wen, S.; Li, Y.; Tian, W. Tetrahedron 2008, 64, 5736. (c)
Kimura, M.; Kuwano, S.; Sawaki, Y.; Fujikawa, H.; Noda, K.; Taga,
Y.; Takagi, K. J. Mater. Chem. 2005, 15, 2393.
(8) Sun, Y.; Xiao, K.; Liu, Y.; Wang, J.; Pei, J.; Yu, G.; Zhu, D. Adv.
Funct. Mater. 2005, 15, 818.
(9) Luo, J.; Zhou, Y.; Niu, Z.-Q.; Zhou, Q.-F.; Ma, Y.; Pei, J. J. Am.
(6) Wang, J.-L.; Duan, X.-F.; Jiang, B.; Gan, L.-B.; Pei, J.; He, C.; Li,
Y.-F. J. Org. Chem. 2006, 71, 4400.
(7) (a) Wang, J.-L.; Yan, J.; Tang, Z.-M.; Xiao, Q.; Ma, Y.; Pei, J. J.
Am. Chem. Soc. 2008, 130, 9952. (b) Wang, J.-L.; Luo, J.; Liu, L.-H.;
Zhou, Q.-F.; Ma, Y.; Pei, J. Org. Lett. 2006, 8, 2281.
Chem. Soc. 2007, 129, 11314.
(10) (a) Dehmlow, E. V.; Kelle, T. Synth. Commun. 1997, 27, 2021. (b)
Elmorsy, S. S.; Pelter, A.; Hursthouse, M. B.; Ando, D. Tetrahedron
Lett. 1992, 33, 821. (c) Terai, H.; Takaya, H.; Naota, T. Tetrahedron
Lett. 2006, 47, 1705.
r
10.1021/ol200227w
Published on Web 03/04/2011
2011 American Chemical Society

several novel truxene-based molecules possessing intri-
guing structures and interesting physical properties.
Scheme 1 presents our synthetic pathway toward
5,10,15-hexaaryltruxenes. Palladium-catalyzed Suzuki
coupling of 1,3,5-tribromobenzene with 2-methylphenyl
boronic acid gave 1,3,5-tri(o-tolyl)benzene (1), which was
previously adopted to synthesize truxene-5,10,15-trione.12
Oxidation of 1 with KMnO4 in the presence of pyridine
afforded the triacid 2 in an isolated yield of 87%. Ester-
ification of 2 with ethanol in the presence of concentrated
H2SO4 gave the triester 3 in a moderate yield (55%).
Double arylations of the triester 3 with various aryl lithium
reagents and subsequent acid-catalyzed intramolecular
Friedel-Crafts cyclizations provided the 5,10,15-hexaar-
yltruxene derivatives 4-6 in high yields (76-85%).
The electronic absorption spectrum of the truxene deri-
vative 4 in CHCl3 revealed an absorption maximum
centered at 313 nm, while the corresponding photolumi-
nescence (PL) spectrum features an emission maximum at
377 nm (Figure 2); π-π* electronic transitions of the
truxene core reasonably account for both features. The
truxene derivatives 5 and 6 exhibited almost the same
photophysical properties as those of 4 (data are summer-
ized in Table S-1, Supporting Information (SI)). It is
generally believed that the saturated bridge units impede
the extension of π-conjugation from the central truxene
core to the aryl peripheral groups. Therefore, the photo-
physical properties should be irrespective of the nature of
the peripheral groups (aryl or alkyl). Notably, however,
our new aryl-substituted truxene derivatives exhibit photo-
physicalcharacteristicsthatdifferfrom thoseoftheiralkyl-
substituted counterparts. For example, the absorption
maximum of the hexa(p-tolyl)truxene 4 was red-shifted
by 6 nm relative to that of C5,C10,C15-hexahexyl truxene
(7); in addition, the PL spectrum of 4 featured a broad
bandlacking vibronic features that was red-shifted by7 nm
relative to that of 7. These results suggest the presence of
subtle orbital interactions between the peripheral tolyl
substituents and the truxene core.
Scheme 1. Synthetic Pathway of 5,10,15-Hexaaryltruxenes
To verify the molecular structures of these products, we
used a two-layer (THF/hexane) crystallization technique
to obtain single crystals of the hexa(p-tolyl)truxene 4 that
were suitable for X-ray diffraction analysis (Figure 1). The
presence of the six p-tolyl groups at the bridge centers C5,
C10, and C15 did not alter the coplanar structural feature
of the truxene core. Two groups of three peripheral p-tolyl
substituents sit on the top and bottom faces, respectively,
of the truxene core; this arrangement would appear to
prevent intermolecular interactions between neighboring
truxene core units.
Figure 2. Absorption and emission spectra of C5,C10,C15-
hexa(p-tolyl)truxene (4) and C5,C10,C15-hexahexyltruxene (7).
We performed theoretical calculations of the truxene
derivative 4 to obtain more detailed insight into its peculiar
photophysical behavior. Figure 3 reveals that the active
moecular orbitals (MOs) of 4 include a non-negligible
fraction from the π- or π*-orbitals of the tolyl groups.
Population analysis using the AOMix program13 indicated
that more than 10% of the population was delocalized to
the tolyl groups in the active MOs listed. In comparison,
a similar analysis of C5,C10,C15-hexaethyltruxene (7a)
(12) Plater, M. J.; Praveen, M. Tetrahedron Lett. 1997, 38, 1081.
ꢀ
(11) (a) Gomez-Lor, B.; de Frutos, O.; Ceballos, P.; Granier, T.;
ꢀ
ꢀ
Echavarren, A. M. Eur. J. Org. Chem. 2001, 2107. (b) de Frutos, O.;
ꢀ
Granier, T.; Gomez-Lor, B.; Jimenez-Barbero, J.; Monge, A.; Gutierrez-
ꢀ
ꢀ
ꢀ
Puebla, E.; Echavarren, A. M. Chem.;Eur. J. 2002, 8, 2879. (c)
ꢀ
ꢀ
Gonzalez-Cantalapiedra, E.; Ruiz, M.; Gomez-Lor, B.; Alonso, B.;
ꢀ
Garcıa-Cuadrado, D.; Cardenas, D. J.; Echavarren, A. M. Eur. J.
´
Org. Chem. 2005, 4127. (d) Luo, J.; Lei, T.; Xu, X.; Li, F.-M.; Ma, Y.;
Wu, K.; Pei, J. Chem.;Eur. J. 2008, 14, 3860. (e) Luo, J.; Lei, T.; Wang,
L.; Ma, Y.; Cao, Y.; Wang, J.; Pei, J. J. Am. Chem. Soc. 2009, 131, 2076.
(13) Gorelsky, S. I. AOMix: Program for Molecular Orbital Analysis,
version 6.4; http://www.sg-chem.net/; University of Ottawa: 2010.
Figure 1. Solid state structure of 5,10,15-hexa(p-tolyl)truxene
(4).
Org. Lett., Vol. 13, No. 7, 2011
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Scheme 2. Synthetic Pathway of New Truxenes 8, 9, and 10
Figure 3. Active frontier MOs of C5,C10,C15-hexa(p-tolyl)-
truxene (4).
When the pH of the solution was increased to 10.5 (by
adding standard 0.01 M NaOH and monitoring using a
pH meter), the intensity of the short wavelength emission
(375 nm) was strongly reduced along with the evident
growth of the long wavelength emission. Thus, prominent
PET occurs between the local excited state originating
from the truxene core of 8 and the electron-donating
peripheral phenoxide units, leading to efficient quenching
of the UV emission. This efficient electron transfer gives
rise to a low-band-gap, charge-separated excited state,
which is intrinsically weakly fluorescent.
Inspired by the elegant syntheses of C3-symmetric cage
molecules by Ahn,14 Gomez-Lor and Echavarren,15 and
Pei,11a we subjected the hexa(m-hydroxyphenyl)truxene
derivative 9 to double-capping with the tribromomesity-
lene compound 10.16 We isolated the cage molecule 11 in
10% yield (Scheme 2). We used a two-layer (hexane/
toluene) crystallization technique to obtain single crystals
of 11 suitable for X-ray diffraction analysis. Figure 4
displays the triple-decker structure of the cage molecule
11 in the solid state.
revealedonly 2-3% delocalization. The delocalizationin4
resulted in a smaller energy gap between its highest occu-
pied MO (HOMO) and lowest unoccupied MO (LUMO),
as well as lower excitation energies [Table S1, SI]. The
truxene core has C3h symmetry; the first two excitations are
symmetry-forbidden (Table S1). This result is consistent
with the large Stokes shift observed between the absorp-
tion and emission spectra; the main absorption peak is
probably due to the optically active S3 state, followed by
internal conversion to the S1 state where emission origi-
nates. For slightly symmetry-broken structures, the S2
state is slightly more allowed than the S1 state in all cases.
Because an interaction exists between the orbitals of the
tolyl groups and truxene core, we might expect that low
frequency torsional modes would perturb this interaction
and modulate the excitation energy; such a situation
would lead to a broad emission spectrum with a much less
vibronic structure.
The PL quantum yields of the truxene derivatives 4, 5,
and 6, measured using an integration sphere (Hamamatsu
C10027), were0.12, 0.02, and 0.02, respectively. Weascribe
the low PL quantum yields of the truxene derivatives 5 and
6, relative to that of 4, to the presence of their electron-rich
methoxyphenyl substituents, which may facilitate photo-
induced electron transfer (PET) to quench the excited
truxene core. PET is strongly dependent on the electron
density of the donor site. In this regard, phenoxide ions,
which are better electron donors than methoxyphenyl
groups, should have a more-pronounced PET effect. To
confirm that PET occurs in hexaphenoxyl-substituted
truxene derivatives, we used BBr3 to demethylate the
truxene derivatives 5 and 6, obtaining the C5,C10,C15-
hexaphenoxyltruxenes 8 and 9, respectively, in excellent
yields (Scheme 2). In CH3CN, the hexa(p-hydroxyphenyl)-
truxene derivative 8 exhibits a PL emission maximum
centered at 375 nm that is associated with a long wave-
length weak emission centered at 550 nm (Figure S1, SI).
Figure 4. Solid state structure of the cage molecule 11 with
hydrogen atoms omitted for clarity; chemical structure of the
core of 11 with atom labeling.
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Org. Lett., Vol. 13, No. 7, 2011

The presence of the two mesitylene caps greatly in-
creased the molecular rigidity, as evidenced by the dis-
torted truxene core of 11 (Figure 4a) relative to that of
the parent counterpart 4 (Figure 1), primarily because
of distortion of the sp3-hybridized carbon bridges
from coplanarity. The average dihedral angle of the
Cb-Cg-Ch-Ci units of 11 was 14.66°. Interestingly, five
of the six methyl groups on the caps are pointed toward the
truxene core, closing the cage walls tightly and allowing
more-compact crystal packing. The distances between the
-OCH2-H hydrogen atoms and the sp2-hybridized car-
bon atoms of the outside phenylene rings of the truxene
core were in the range from 2.7 to 3.6 A, implying the
two capping units inhibits the free rotation of the aryl
substituents, thereby decreasing the possibility of interac-
tion between peripheral aryl groups and the truxene core.
Indeed we observe a smaller population in the peripheral
arylsubstituentsof 11inthe activeMOs, relativetothatfor
4 (Table S2). The calculated excitation energy of the cage
molecule 11 is slightly blue-shifted from those of 4 and 6,
probably due to similar conformational constraints. The
lack of peripheral interference means that the truxene unit
in the cage molecule 11 exhibits photophysical behavior
similar to that of the hexaalkyltruxene derivative 7. Inter-
estingly, for the “open” hexa(p-tolyl)truxene derivative 4
we observed (both experimentally and computationally)
significant interactions between the aryl groups attached
to the sp3-hybridized bridge centers and the truxene
core, mediated through σ-bonds. When we extended the
substituent framework further to give the “closed” truxene
derivative 11, however, the walling and capping aryl
groups did not interact significantly with the core unit’s
π-orbitals. Thus, it is possible to fine-tune the electronic
properties of a molecule through σ-bond connection to
aryl substituents, but there is a limit to such mediation
effects. In our case, layers of capping aryl groups inhibited
any further photophysical communication.
In summary, we have established a new synthetic route
toward truxene derivatives, providing the flexibility to
introduce aryl groups at the saturated bridge carbon
atoms. Through experimental observations andtheoretical
calculations, we also confirmed that σ-bond-separated aryl
units could influence the physical properties of the truxene
core. Capping of the peripheral aryl groups led to the
intriguing triple-decker cage compound 11, which pos-
sessed a distorted truxene core. The ability of the periph-
eral aryl units to perturb the photophysical properties of
the truxene core was mitigated in the cage compound
because of its molecular rigidity.
possibility of weak C-H π interactions. Our calcula-
3 3 3
tions suggest that this conformation is 17 kcal/mol more
stable than the one in which all of the methyl groups point
away from the truxene core. The existence of such hydro-
gen bonding interactions may also account for the outside
phenylene rings of the truxene core having twisted away
from coplanarity, since the distance of the outside pheny-
lene rings to the CH bonds are shortened in many cases.
The central benzene ring of the truxene core and the
mesitylene rings of the caps are separated with centroid-
to-centroid distances of 5.54 (top to center) and 5.25 A
(bottom to center), indicating no evident π-π interactions,
in agreement with our calculation of a negligible popula-
tion of the caps of 11 in the active MOs (Table S1).
Acknowledgment. This work was financially supported
by the National Science Council, Taiwan. We thank
Professor Jian Pei (Pekin University) for donating the
model compound C5,C10,C15-hexahexyl truxene (7) .
Figure 5. Absorption and emission spectra of the C5,C10,C15-
hexaaryltruxene 6 and the cage molecule 11.
Supporting Information Available. Detailed experi-
mental procedures; spectroscopic characterization;
photophysical data of new compounds; PL spectra of
truxene derivative 8 at various values of pH; computa-
tional details; population analysis for the frontier MOs
The molecular rigidity of 11 has significant effects on its
photophysical properties (Figure 5). This cage molecule
exhibits an emission signal with fine vibronic structures
that is slightly blue-shifted relative to that of its parent
compound 4; the blue shift is consistent with a lack of
coplanarity for the truxene core of 11 and results in a
slightly increased band gap. Moreover, the presence of the
1
of 4, 7a, and 11; H, 13C NMR spectra of new com-
pounds; and cif files of compounds 4 and 11. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(14) Kim, J.; Kim, Y. K.; Park, N.; Hahn, J. H.; Ahn, K. H. J. Org.
Chem. 2005, 70, 7087.
ꢀ
(15) Gomez-Lor, B.; Hennrich, G.; Alonso, B.; Monge, A.; Gutier-
(16) Cho, B. R.; Chajara, K.; Oh, H. J.; Son, K. H.; Jeon, S.-J. Org.
Lett. 2002, 4, 1703.
rez-Puebla, E.; Echavarren, A. M. Angew. Chem., Int. Ed. 2006, 45, 4491.
Org. Lett., Vol. 13, No. 7, 2011
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